1. Field of the Invention
The present invention relates to an exposure method, used with a projection exposure apparatus, for fabricating semiconductor devices, image sensing devices (such as charge-coupled devices), liquid crystal displays, thin film magnetic heads or the like. More particularly, it relates to an exposure method suitable for use with a projection exposure apparatus which is equipped with an auto-focusing mechanism for positioning the surface of a photosensitized substrate to cause it to be coincident with an image plane of a projection optical system of the projection exposure apparatus.
2. Description of the Related Art
In photolithographic process for fabricating semiconductor devices or other products, a projection exposure apparatus (such as a stepper) is commonly used in which an image of a pattern on a mask or reticle is transferred through a projection optical system onto each of a plurality of shot areas defined on a photoresist-coated wafer (or other substrate such as a glass plate). In a projection exposure apparatus, the wafer may be held against a top surface of a wafer holder utilizing vacuum, and the top surface of the wafer may have a relief pattern of concentric circles. In addition, such a projection exposure apparatus may include an auto-focusing mechanism comprising an auto-focus sensor system and a stage mechanism. The auto-focus sensor system detects a focusing position (or vertical position) of the surface of the wafer at a measurement point which may be selected, for example, to be at the center of an exposure field of the projection optical system. The stage mechanism uses the detection results from the auto-focus sensor system so as to position the surface of the wafer in each shot area to locate the surface close to and within a range of focal depth about an image plane of the projection optical system.
In a prior art auto-focusing mechanism of this type, calibration is achieved such that the auto-focus sensor system provides a detection signal at a predetermined reference level when the surface of the wafer is coincident with the image plane of the projection optical system. Further, in order to maintain focusing during an exposure operation by an auto-focus technique, a servo-control technique is used to control the focusing position of the wafer through the stage mechanism such that the detection signal from the auto-focus sensor system remains substantially at the reference level.
As described, in this prior art auto-focusing system, the focusing position of the wafer at a preselected measurement point in the exposure field is caused to be coincident with the image plane whose position is known. Thus, if there is a protuberance of the surface of the wafer at a position near the measurement point in the shot area to be exposed, which may occur due to the intrusion of foreign matter, such as a residue of photoresist or a dust particle, between the bottom surface of the wafer and the top surface of the wafer holder, then only the region of the protuberance may be made coincident with the image plane with the result that the remaining large portion of that shot area is positioned unacceptably far from the image plane. Consequently, the chip pattern formed in that shot area will be defective. However, until now, it has been impossible to detect any such defective chip patterns caused by foreign matter before checking the integrity of chip patterns on wafers for after the entire fabrication process has been completed.
If such foreign matter can be detected, useless exposure operations may be eliminated. Specifically, if large foreign matter is detected in a particular shot area during the exposure process for the first level layer on the wafer, exposure operations can be omitted for that shot area during the exposure processes for the second and any subsequent level layers.
Nevertheless, adding a detection device specifically designed for foreign matter detection purpose to the projection exposure apparatus would result in a complication of the arrangement of the wafer stage and hence in an increase in the manufacturing costs.
In view of the foregoing, it is an object of the present invention to provide an exposure method which is capable of detecting foreign matter existing on the bottom of a wafer without the need for any special detection device.